1. Field of the Invention
The present invention relates to a method for fabricating an optical device composed of two prisms bonded together, and also relates to a joining jig for use in the fabrication of such an optical device.
2. Description of Related Art
Today, following the advent of Blu-ray Disc and HD-DVD, prisms (for example, cubic joined prisms used as polarizing beam splitters) for use in optical pickups are required to have highly accurate exterior dimensions. For example, as shown in FIG. 16, whereas conventionally the tolerance for exterior dimensions such as vertical and horizontal widths is within about ±0.1 mm of the nominal dimensions, nowadays it is more often within ±0.05 mm of the nominal dimensions.
Moreover, in pickup prisms, as shown in FIG. 17, the deviation of the optical axis of the reflected light relative to the optical axis of the incident light—called the beam shift—is often required to be within ±0.05 mm. The beam shift results chiefly from a deviation of the junction surface (reflective surface) between two prisms. Specifically, if the junction surface deviates to one of the positions indicated by broken lines in FIG. 17, a beam shift arises in the emergent light.
Thus, today, pickup prisms are required to have accurate exterior dimensions and a small beam shift at the same time.
Against this background, there have conventionally been proposed various methods for fabricating pickup prisms. For example, according to the method disclosed in JP-A-2000-143264 (see, in particular, claim 1 and FIG. 3), as shown in FIG. 18, plates of a glass material having a beam-splitting optical thin film applied over them are laid on top of another with an optical adhesive (for example, an ultraviolet-curing adhesive) into the shape of stairs, and are then processed into pieces having the desired cubic shape through a procedure consisting of the steps of cutting, polishing, anti-reflection film coating, laying-together, cutting, polishing, and cutting.
With this method, it is possible to use plates of a glass material, which are relatively inexpensive; in addition, it is possible to fabricate a large number of prisms through a sequence of processing steps. Thus, it is possible to fabricate prisms at low cost. On the disadvantageous side, the method involves repeated laying-together, cutting, and polishing, and is thus liable to suffer from accumulated processing errors. As a result, as shown in FIG. 19, it is difficult to form the junction surface 101—the accuracy of the position of which affects the beam shift—at a position accurate enough to achieve the tolerated beam shift.
Another conventionally proposed method for fabricating pickup prisms employs long-size triangular prisms. According to this method, as shown in FIG. 20, first, two long-size triangular prisms 201 and 202 are prepared. Of these prisms, one has a beam-splitting optical thin film and an anti-reflection optical thin film applied over it, and the other has an anti-reflection optical thin film applied over it. Next, the long-size triangular prisms 201 and 202 are joined together with an optical adhesive (for example, an ultraviolet-curing adhesive), and are then pressed into a V-shaped groove 203a in a joining jig 203 so that the adhesive is cured with the 45-degree vertices at the top of the junction surface put together under a pressure. Thereafter, the long-size prism 204 thus obtained is cut into discrete prisms having a cubic shape.
With this method, as shown in FIG. 21, by controlling the height of the long-size triangular prisms 201 and 202 within ±0.035 mm of the nominal dimension, it is possible to obtain prisms with the tolerated beam shift (within ±0.05 mm) (in actual fabrication, for an ample margin of safety, the height of the long-size triangular prisms is controlled more strictly). Here, when one of the obtained cubic prisms is assembled into an optical pickup, it is positioned with positioning pins 205 placed in contact with the shorter sides—those intersecting at right angles—of one of the two triangular prisms joined together, more specifically the one having the beam-splitting optical thin film formed on it. In this way, it is possible to easily control the beam shift within the range mentioned above.
The trouble here is that, as shown in FIG. 22, the long-size triangular prisms 201 and 202 generally have their 45-degree ridges (corners) chamfered to about C0.1 to C0.2 for crack prevention. Here, chamfering to about C0.1 to C0.2 denotes cutting a corner off at a position of 0.1 to 0.2 mm from the edge. A variation in the chamfering here causes a deviation in the joining of the long-size triangular prisms 201 and 202 under a pressure. This increases the difference between the vertical and horizontal widths of the eventually obtained cubic prisms, and thus makes it difficult to obtain the desired accuracy in exterior dimensions.